The manufacture of high density microelectronic packages and devices, such as integrated chip packages (ICPs) or printed circuit boards for cell phones, requires the fabrication of single or multilayer printed circuit boards (PCBs) having high density arrays of small holes referred to as microvias. Microvias are formed using sophisticated mechanical and laser drilling systems and, once formed are plated to provide electrical connections and leads to and between the layers, circuits and circuit elements of single and multi-layer PCBs.
Over the last several years the number of microvias that can be and are formed into a single layer or a single or multi-layer PCB has increased dramatically and it is now conceivable to have several hundreds of thousands of microvias in a single layer of a PCB used to manufacture ICPs. Since microvias provide the electrical or communication link or links to each layer or level of a PCB or multi-layer PCB it is critical that these small holes are processed and fabricated correctly to fulfil their intended purposes and functions. The fabrication of vias, however, is subject to numerous potential defects, such as missed holes, blocked or partially drilled holes, over or under sized or mis-shapen holes, liftoff, fracture or failure of the printed circuit pads surrounding the vias, often due to the ablation of pads by the microvia hole drilling process, and the presence of organic and other residues or smears on the pads due to inadequate de-smearing and cleaning processes, including residues or smears of transparent substances that are very difficult to detect.
The inspection of single or multi-layer PCBs, and in particular the inspection of microvia holes and microvias at various points in the fabrication process is therefore critical to the fabrication of PCBs and ICPs. The inspection of vias is typically automated and several illumination and inspection techniques using flood illumination and automated camera frame grabbers and analysis software have been employed to inspect these partially drilled microvias, typically <100 μm (˜0.004″) in diameter and with aspect ratios of 1:1 to 1:3, hole diameter to depth, by imaging and analyzing successive fields of view across the PCB.
The methods of the prior art, however, inherently provide only low magnification of the fields of view, however, resulting in a relatively low number of image pixels in each image and a lack of resolution as regards any features, such as microvias or portions of microvias, appearing in an image. To illustrate, and as shown in FIGS. 1A and 1B, the methods of the prior art could only image one field of view over the entire array of pixels for the camera. The magnification and image resolution of the system is limited by the separation between the features being inspected, since the single field of view magnifies both the features of interest and increases the separation between the features shown in the image. The choice is thereby between an imaging system such as illustrated in FIG. 1A that images a larger field of view, that is, a larger area of a PCB, but at a lower resolution, or an imaging system such as illustrated in FIG. 1B that has higher magnification to resolve smaller features but at the cost of a correspondingly smaller field of view that often will not include all of the features of interest. The ability of the methods of the prior art to detect flaws and problems of various types is thereby limited, because pitch separation increases with magnification. The limits on image resolution in turn result in a significant possibility that defects, sometimes a majority of the actual defects, are not detected, which in turn results in unacceptable or undesirably low yields from the fabrication process.
Another problem with the methods of the prior art, and in particular scanning systems having wide fields of view and using retro-reflected laser illumination together with fluorescence emission or flood illumination, is that a large part of the illumination energy and the subsequent image scanning process is expended on relatively large areas that do not include microvias. The illumination and scanning of areas that are, in fact, not of interest thereby undesirably wastes a significant part of the system energy emission and processing throughput.
It must also be noted with regard to these problems of the prior art that these methods of the prior art are not readily scalable, so that these problems of the prior art become rapidly more severe as the microvias are reduced in size and as higher and higher microvia and circuit densities are achieved in PCBs.
To illustrate, current AOI systems cannot be increased in resolution, that is, cannot use higher image magnification, because the resulting increase in image processing load would unacceptably reduce the inspection throughput of the systems. The effective decrease in image feature resolution per pixel, however, is significantly decreased as the microvias become smaller and smaller, that is, each pixel represents a proportionally larger portion of the microvia image as the sizes of the microvias is reduced. The decrease in image resolution with decreasing microvia size, however, proportionally decreases the probability that a given feature of a microvia, such as a fault or a contaminating object or objects, can or will be adequately represented and detected in the image. For example, since the field of view of the imaging mechanism is fixed, a reduction in microvia size from 0.65 μm to 0.25 μm results in a reduction of more than half in the pixel resolution of the image. Since defects on the order of 2 μm can be problematic, the current systems of the prior art will thereby fail to achieve the required level of quality control necessary to ensure high yields on such products as chip packages which utilize <+25 micron diameter microvias.
This problem will continue to become more and more severe as PCBs Boards become denser and denser with potential microvias densities of >300 k per panel layer and forseeable densities approaching 500 k microvias per panel.
The present invention provides solutions to these and other problems of the prior art.